BRACHIOPODA. 441 



families the arms are supported upon a shelly loop, of 

 variable shape and size ; whilst in some of the extinct 

 Rhynchonellidce, and in the Spiriferidce, the arms were sup- 

 ported by large spirally-coiled calcareous lamellae. 



The Brachiopods are of such importance as fossils that the 

 characters of the more important groups, with their geological 

 range and leading genera, will here be given, along with figures 

 of the commoner types. As regards the general distribution 

 of the class in time, the Brachiopoda are found from the Cam- 

 brian rocks up to the present day, and present us with an 

 example of a group which appears to be slowly dying out. 

 Nearly four thousand extinct species have been described, 

 and the class appears to have attained its maximum in the 

 Silurian epoch, which is, for this reason, sometimes called the 

 "Age of Brachiopods." Numerous genera and species are 

 found also in both the Devonian and Carboniferous forma- 

 tions. In the Secondary rocks Brachiopoda are still abundant, 

 though less so than in the Palaeozoic period. In the Tertiary 

 epoch a still further diminution takes place, and at the pres- 

 ent day we are not acquainted with more than a hundred 

 living forms. Of the families of Brachiopoda, the Productidce, 

 Strophomenidce, and Spiriferidce are the more important extinct 

 types. Of the genera, the most persistent is the genus Lin- 

 gula, which commences in the Cambrian rocks, and has main- 

 tained its place up to the present day, though it appears to 

 be gradually dying out. 



According to Woodward : " The hingeless genera attained 

 their maximum in the Palaeozoic age, and only three now 

 survive (Lingula, Discina, Crania) the representatives of as 

 many distinct families. Of the genera with 'articulated valves, 

 those provided with spiral arms appeared first, and attained 

 their maximum while the Terebratulidce were still few in num- 

 ber. The subdivision with calcareous spires disappeared with 

 the Liassic period, whereas the genus Ehynchonella still exists. 

 Lastly, the typical group, Terebratulidce, attained its maxi- 

 mum in the Chalk period, and is scarcely yet on the decline." 



Of the families of the Brachiopoda, the Productidce and 

 Strophomenidce are exclusively Palaeozoic. The Spiriferidce 

 are mainly Palaeozoic, but extend into the Lias, where they 



